Apparatus for reversing colour images



Dec. 20, 1966 H. MEY 3,292,486

APPARATUS FOR REVERSING COLOUR IMAGES Filed Dec. 4, 1963- INVENTOR y b Ham3 uer3 Meg .71 5 Wm, JWK. Pod ATTORNEY-S United States Patent Ofifice 3,292,486 Patented Dec. 20, 1966 3,292,486 APPARATUS FOR REVERSING COLOUR IMAGES Hansjuerg Mey, Oberengstringen, Switzerland, assignor to Gretag Aktiengeselischaft, Regensdorf, Switzerland Filed Dec. 4, 1963, Ser. No. 327,939 Claims priority, application Switzerland, Dec. 7, 1962, 14,419/62 12 Claims. (Cl. 88-24) This invention relates to an apparatus for reversing colour images.

Methods of and apparatus for reversing colour images are used particularly for judging colour negatives or for printing and enlarging colour diapositives on normal colour negative material.

For the production of coloured pictures on paper, use is preferably made of photographic negative material which can be printed or enlarged on colour paper in the simplest manner by the negative-positive process. This method gives the best results, but has the disadvantage that judgement of the negative can give only with great difiiculty an impression of the positive (paper picture) to be expected. In the case of black-and-white negatives, judgement is easier inasmuch as a reversal of brightness can be imagined after some practice. Experience has shown however that in the case of colour negatives it is not possible in practice to do so. This is probably due to the following reasons: In the first place, in the case of a colour negative not only the brightnesses but also the colours are reversed (to their complementary colours); secondly, the absorptions of the negative dyestuffs are not adapted to the eye but to the sensitisation of the colour paper. In order to achieve good separation of the three printing colours, the three dyestuffs absorptions must be separated as far as possible apart in the spectrum. This has the consequence that all blue and red negative colour fractions are displaced towards the edge of the spectrum of the human eye, and a negative appears purple even if it reproduces a neutral grey tone. In the third place, in certain makes of film, coloured masks are incorporated in the negative film, which impart a completely different colour (for example intense purple) to the entire negative.

Judgement however is extremely important precisely in the case of negative colour material. This is important on the one hand to the amateur who must decide which pictures are to be enlarged, and on the other hand to the photographic laboratory which must be able to deduce from the negative the exposure times and filter data for the enlarging operation. Incorrect judgement leads to prints which have a colour cast and which must be re-made, thus increasing the cost of the entire process.

Proposed methods of image reversal are for example infrared extinction or a television type of splitting and reproduction of the image in fine lots by means of electric signal reversal. While the first-mentioned method is not suitable for the reversal of coloured images, the second-mentioned method is extremely expensive. Instruments working on theelectric signal reversal principle have already been built. These instruments have a colour television screen (monitor) on which an image can be produced which corresponds to the picture to be expected subsequently on paper. The extremely great expenditure for electronic equipment however prevents wider application of such instruments.

In order to avoid the above-described disadvantages of previously proposed image reversal methods or apparatus the present invention proposes to use xerographic means for the reversal of colour images.

Xerography is in itself known generally. This method of reproduction consist essentially in that the image to be copied is projected onto an electrostatically charged photographic semiconductor layer and the latter then has a likewise electrostatically charged coloured powder strewn over it. Since the individual parts of the photographic semiconductor layer are charged to a greater or lesser extent in accordance with their exposure (lighttime integral) more or less powder remains adhering to the various parts of the surface. By corresponding selec tion of polarity and colour (black-and-white) of plate and powder, a reversed powder image of the original to be printed can be obtained on the plate. In the known xerographic method, this powder image is then transferred to a sheet of paper. For this purpose the sheet of paper is placed on the plate and electrically charged to constitute an image-former. The powder attracted by the charged paper is then fixed by a heating process. In accordance with one aspect of the present invention there is provided a method of reversing colour images, comprising the steps of producing optical colour separations of a colour image, producing xerographic reversed images from said colour separations, and reproducing these xerographic partial images superimposed on one another and in correct register. The term colour images as employed in this specification is to be understood to include both negatives and positives.

The present method therefore starts with reversed images which can be produced on a photographic semiconductor layer. The semiconductor layer may be sensitised for a determined spectral range, or in the case of panchromatism may be restricted by means of filters to a determined spectral range. By adapting this spectral range to the spectral absorption of a negative dyestuff, the effect may be achieved that the photographic semiconductor layer will be influenced practically only by a single colour layer in the negative film; the positive obtained by powder development is then a colour separation positive of the colour negative original. The same applies to the adaptation of the spectral range to the spectral absorption of a positive dyestuif. In this case the powder image is a colour separation negative of the colour positive original. These xerographic colour separation positives are generally black-and-white images.

When the present method is applied to the reversal of colour negatives, the xerographic partial images could be projected with colourless light. In this case blackand-white positives of the coloured negative original would be obtained.

According to a further features of the invention, however, for each xerographic partial image, the colour of the projection light is so selected that this colour coincides at least approximately with the primary colour of the light which was used to obtain this partial image. In the reversal of a colour negative, the xerographic partial image corresponding to the yellow separation of the negative will be projected with blue light, the partial image corresponding to the magenta separation will be projected with green light, and the partial image corresponding to the blue-green separation will be projected with red light. This will produce a positive colour image of the negative original capable, for example, of being made visible on a ground glass screen and used for judging the picture.

A modification of the present method consists in that in the reversal of colour positives the xerographic partial images are projected with correct register onto colour negative material. Negative material exposed in this manner can then be developed into a positive of a positive original.

As already mentioned, in previously known xerographic reproduction methods, the powder image is transferred to a paper carrier and fixed on the latter by fusing or the like. Resolution losses naturally occur when this is done. In carrying out the present method, this disadvantage may be avoided by carrying out the reversal process on powder lying loose on the photographic semi-conducting layer. The powder images to be projected therefore need neither be transferred to a paper carrier or the like, nor fixed. After utilisation, the powder can easily be removed from the semiconductor layer, so that not only the same semiconductor layer but also the same powder can be repeatedly re-used.

According to another aspect of the present invention there is provided apparatus for reversing a colour image, comprising a light source for illuminating the colour image, an optical system comprising a plurality of light splitters for separately projecting a respective colour separation of the colour image, an electrostatically chargeable photographic semiconductor layer constituting an image plane for each of said colour separations, means for electi'ostatically charging said layer, means for developing a reversed xerographic partial image on said layer, and means for projecting into a second image plane said partial images of the individual colour separations superimposed on one another and in correct register. Dichroic mirrors are preferably used as light splitters. However, semi-transparent mirrors followed by colour separation filters may also be used.

In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawing which is a perspective view illustrating diagramatically and by way of example apparatus for reversing colour images in accordance with the present invention.

Referring now to the drawing the apparatus includes a white light source 1 arranged to illuminate through a condenser system 2 a colour picture 3a (colour negative or colour positive) which is disposed in a plane 3, and which in the present case is transparent, so as to reproduce the picture through an objective 5. The path of the image-forming rays runs through a first dichroic mirror 6, which for example reflects the blue light fraction up to a wavelength of 500 m but on the other hand allows the green and red light fractions to pass. The light rays passing through the mirror 6 pass on to a second dichroic mirror 7, which reflects for example the green light fractions up to a wavelength of 600 m so-that finally only the red light fractions above 600 mu impinge on an ordinary surface-coated mirror 9. A deviating mirror 12 reflects the blue light fractions onto the plane of the image, whereby a sharp delineation of the picture original 3a disposed in the plane 3 is produced in region 16. Correspondingly, an image is formed in region 14 with the green light fractions through a deviating mirror 8 and in region 15 with the red light fraction through a deviating mirror 10. The arrangement, particularly the adjustment of the mirror systems 67912-810 is so disposed that the three partial images of the single picture original 3a disposed in the plane 3 lie in the same image plane.

The two dichroic mirrors 6 and 7 together form a light splitter, which divides roughly into three spectral ranges the light from the lamp 1 after it has been coloured by the picture original. Finer adaptation of the three light fractions to the spectral absorption of the picture original 3a is achieved with three colour filters, each of which is inserted into one image ray path: a green filter 28, a red filter 29, and a blue filter 30. These three filters are grouped together in a common filterplate 11.

The three images produced in the regions 14, 15 and 16 are therefore the colour separations of the colour picture original disposed in the plane 3, which are to be reversed with the aid of xerographic means. For this purpose the image plane consists of a photographic semi-conducting layer 13 provided on a conducting carrier, for example a zinc oxide layer on an aluminium foil. The photographic semiconducting layer 13 is sensitised for the entire spectrum utilised, that is to say the spectrum determined by all three negative or positive absorptions of the picture original.

In the illustrated apparatus, the carrier is an endless flexible band 17 of conducting material, which is guided around at least two guide rollers 19 and 20 and on the outer side of which the photographic semiconducting layer 13 is provided, at least one of the guide rollers 19, 20 being adapted to be driven mechanically.

The band 17 or the photographic semiconducting layer 13 provided on said band passes beneath a charging device before entering the projection zone. This charging device consists of a corona discharge device 21, which is fed by a high tension source 22, while the carrier band 17 is earthed through the guide roller 19.

For the development of the latent electrostatic charge images which can be produced on the photographic semiconducting layer, a developer slide 23 slidable on rails 27 is provided, and is equipped with a magnetic brush of known type, formed by a magnetised roller 24. The slide 23 can be moved over the semiconductor layer 13 from the right-hand position shown in the drawing, in which it is situated above a container 25 holding developer powder and open at the top. During this movement of th slide 23, the roller 24 can be rotated. In the position shown in the drawing the rotating roller 24 withdraws developer powder from the container 25, said powder accumulating on the periphery of the roller or on iron powder particles situated there, thus being charged by triboelectrical effect. If the slide 23 is now guided above the photographic semiconductor layer 13, with the periphery of the roller 24 almost in contact with said layer, the individual parts of the surface of the photographic semiconductor layer attract more or less developer powder in accordance with their condition of charge.

The partial images of the individual colour separations which can be produced on the surface of the photographic semiconductor layer can now be projected into the reproduction plane 31 with correct register. According to a further feature of the invention, and in accordance with the apparatus illustrated, the arrangement is such that the image forming system for the xerographic images lies in the reciprocal ray path of the image forming systems 5, 6, 7, 9, 12, 8 and 10 of the colour original. At the same time the light source can also be used, after removal of the picture original 3a, for projecting the xerographic partial images. The necessary beam splitting can be achieved by disposing a semi-transparent deviating mirror 4 between the image forming optical system 5 and the light source 1. Consequently a part of the light from the light source 1 which is remitted from the xerographic images passes into the image plane 31. During the projection of the picture original 3a the mirror 4 is preferably removed from the ray path.

Increased remission of the powder images into the solid angle utilisable for their projection is achieved by forming the photographic semiconductor layer 13 of transparent, preferably optically clear material, while the carrier surface facing the photographic semiconductor layer 13 either has a surface reflecting at the restricted solid angle, or this surface of the carrier is formed by a retroflective material which reflects. in itself the impinging light rays.

It is naturally also possible to effect the exposure of the photographic semi-conductor layer and the subsequent projection of the powder images by means of different light sources. In this case transparent, preferably optically clear material will be selected both for the photographic semiconductor layer and for its carrier and the illuminating optical system for the projection of the powder images will be disposed on the opposite side of the powder image to the image forming optical system 5. This gives very good light efficiency. For this purpose the carrier used may be a film which is coated at least on the side carrying the photographic semiconductor layer with a conductive transparent coating.

Another possible way of projecting the powder images with a separate source of light consists of a dark field arrangement, in which the projection light source is disposed laterally and .only slightly above the powder images. On the preferably specular semiconductor layer the projection light rays are reflected away out of the angular range of the image forming optical system, While the image parts coated with powder produce stray light which falls intothe angular range used and iscollected by the objective 5.

With the aid of the arrangement illustrated, the reversal of a colour negative takes place for example in the following mannerz' Before exposure the band, 17 coated on its outer side with ithe photographic semiconductor layer 13 is moved in the direction of the arrow 18 such a distance that the portions of the band which at the beginning of the movement are. situated above the roller 19 located on the left in the drawing come to lie over the right-hand roller 20. Atthe same time the photographic semiconductor layer is moved past under the corona discharge device 21 which is switched on at the commencement of the movement, so that at the end of themovement of the band the entire region of the photographic semiconductor layer 13 coming to lie in the image plane of the picture original is charged. The separation negativesare now exposed onto the photographic semiconductor layer 13. The developer slide 23 is then moved over the photographic semiconductor layer 13 (fromright to left in the drawing). The latent electrostatic charge image is thereby developed into a powder image. After the return of the developer slide 23, three positive powder images are left on the photographic semiconductor layer 13 in the regions 14, 15, and 16.

The negative (3a) is now pushed out of the path of the rays, so that the powder images are each homogeneously illuminated by the white light source 1 in their corresponding primary colours through the exposure optical system. The light remitted by the powder images passes back through the same exposure optical system with reciprocal ray paths. In the plane 31, which is the same distance from the objective 5 as the negative plane 3, an additive mixing of the three positive powder images, each in its primary colour, is effected through the semi-transparent mirror 4, that is to say a coloured diapositive is formed. The image plane 31, which is for example provided with a ground glass screen, is the viewing plane. Geometric distortions of the image-forming optical system are eliminated, since the path of the rays runs through the optical system in both directions if only the semiftransparent mirror 4 is optically fiat.

The visible positive is therefore a colorimetrically purely additive mixed product of the three colour separation positives. This means that the projection primary colours must form an additive triplet. The filter plate 11 with the filters 28, 29, and 30 should be replaced by a different filter disc with a new filter triplet for the projection of the positive, because the exposure filters 28, 29 and 30 are adjusted to the negative colorants and do not correspond to the additive primary colour conditions. In the simplest case the filters 28, 29, 30 could be simply omitted in the positive projection, since the dichroic mirrors 6 and 7, which are passed through twice during the projection, in any case already produce primary colours similar to optimum colours, with the limits 500 and 600 m indicated as an example. By the use of a correcting projection filter set (for example ideal colour filters) the saturation of the colours may be increased, but only at the expense of their brightness.

When the judgment of the positive visible in the plane 31 has been completed, the band carrying the photographic semiconductor layer is moved once again, as described above. A photographic semiconductor layer homogeneously acted on by the corona discharge device is thereby brought into the image plane once again. In addition, on this movement of the band the powder which produced the previous xerographic images is swept off by a stationary brush 26 and drops into the storage container 25, where it restores to the normal value the remaining developer powder mixture. A mixing device (not illustrated) ensures the homogeneity of the powder mixture, which is thus ready for the next development.

For the reversal of a colour positive the procedure is fundamentally the same as described above for a negative. It is merely necessary to select, in accordance with the different spectral absorptions of the diapositive colour layers and in accordance with the spectral sensitisations of the photographic paper, filters different from those used in the case of negative conversion.

Through the disposal of the photographic semiconductor layer on an endless band, apart from the simplicity of the charging and extinguishing device, the effect is achieved that a fresh semiconductor layer is always available for exposure. This is important inasmuch as a layer one exposed will return to its state of rest only after a certain time, this being known as dark adaptation. Depending on the length of this recovery time, the endless band may also be guided over more than two guide rollers. The mean utilisation time of the same part of the layer is thereby reduced and the rest time increased.

In order to improve half-tone properties, the image can be exposed through a mosaic screen, in which case the gradation can be varied within wide limits by means of control diaphragms in the case of optical mosaic screening. The fact that it is possible to enlarge the xerographic image as compared with the picture original and the visible reversal image, facilitates the production of a fine mosaic screen on the viewing side, with good resolution. An improvement in the half-tone properties can also be achieved by projecting not merely three images, but for example three images for each primary colour, that is to say a total of nine images, the partial images of each primary colour being exposed with an increasing light-time integral. The additive superimposition of these individual images gives a variable gradation with better half-tone properties. Through the mixed utilisation of negatively and positively developing powders, or the mixed utilisation of positive and negative charge polarities, it is even possible to obtain masking effects.

The present image reversal process may be carried out both by using black powder and white photographic semiconductor material, and by using white powder and black photographic semiconductor material. The polarity of powder and plate should be the same in the first case, and opposite to one another in the second case.

The previously mentioned dark field arrangement is best achieved with white powder on a specular photographic semiconductor surface or specular support with transparent photographic semiconductor, and with opposite polarities of powder and plate.

Zinc oxide, which has previously been mentioned, amorphous selenium, or the like may be used as photo graphic semiconductor materials.

What is claimed is:

1. Apparatus for reversing a colour image, comprising a light source for illuminating the colour image, an optical system comprising a plurality of light splitters for sep arately projecting a respective colour separation of the colour image, an electrostatically chargeable photographic semiconductor layer constituting an image plane for each of said colour separations, means for electrostatically charging said layer, means for developing a reversed Xerographic partial image on said layer and means for projecting into a second image plane said partial images of the individual colour separations superimposed on one another and in correct register.

2. The apparatus of claim 1, wherein the image-forming system for the xerographic partial images lies in the reciprocal ray path of the image-forming system of the colour original.

3. The apparatus of claim 1, wherein additional light splitters which can be brought into the partial ray paths are provided.

4. The apparatus of claim 1, wherein the light splitters are dichroic mirrors.

5. The apparatus of claim 1, wherein correcting filters are provided after the light splitters.

6. The apparatus of claim 1, wherein the light splitters are semi-transparent mirrors followed by colour separation filters.

7. Apparatus for reversing a colour image, comprising a light source for illuminating the colour image, an optical system comprising a plurality of light splitters for separately projecting a respective colour separation of the colour image, filter means insertable into the light paths of said colour separations, an endless flexible band of conducting carrier material, at least two guide rollers over which said band is guided, an electrostatically chargeable photographic semiconductor layer disposed on the outside of said band and arranged to constitute a common image plane for all said colour separations, means for driving at least one of said guide rollers mechanically, an electrostatic charging device for charging said layer as the latter passes under the device, a developer slide movable over said layer to develop a reversed xerographic partial image thereon, and means for projecting into a second image plane said partial images of the individual colour separations superimposed on one another and in correct register.

8. The apparatus of claim 7, wherein the photographic semiconductor layer is formed of transparent optically clear material.

9. The apparatus of claim 7, wherein the carrier surface which faces the photographic semiconductor layer has a surface adapted to reflect at a restricted solid angle.

10. The apparatus of claim 7, wherein the surface of the carrier is formed by a retroflective material which at least substantially reflects in itself impinging light rays.

11. The apparatus of claim 7, wherein the carrier is formed of transparent optically clear material.

12. Apparatus for reversing a colour image, comprising a light source for illuminating the colour image, an optical system comprising a plurality of light splitters for separately projecting a respective colour separation of the colour image, an electrostatically chargeable optically transparent photographic semiconductor layer constituting an image plane for each of said colour separations, means for electrostatically charging said layer, means for developing a reversed xerographic partial image on said layer, a semi-transparent deviating mirror interposable between the image-forming optical system for the colour image and said light source, at least one projection light source disposed beneath the layer and an imageforming optical system for the xerographic partial images lying in the reciprocal path of the image forming optical system for the colour image, whereby said partial images can be reproduced superimposed on one another and in correct register.

References Cited by the Examiner UNITED STATES PATENTS 2,986,466 5/1961 Kaprelian 88-245 X 3,121,375 2/1964 Fotland et al 1.7 3,215,030 2/1965 Jordan 88 -24.5

NORTON ANSHER, Primary Examiner. R. A. WINTERCORN, Assistant Examiner. 

1. APPARATUS FOR REVERSING A COLOUR IMAGE, COMPRISING A LIGHT SOURCE FOR ILLUMINATING THE COLOUR IMAGE, AN OPTICAL SYSTEM COMPRISING A PLURALITY OF LIGHT SPLITTERS FOR SEPARATELY PROJECTING A RESPECTIVE COLOUR SEPARATION OF THE COLOR IMAGE, AN ELECTROSTATICALLY CHARGEABLE PHOTOGRAPHIC SEMICONDUCTOR LAYER CONSTITUTING AN IMAGE PLANE FOR EACH OF SAID COLOR SEPERATIONS, MEANS FOR ELECTROSTATICALLY CHARGING SAID LAYER, MEANS FOR DEVELOPING A REVERSED XEROGRAPHIC PARTIAL IMAGE ON SAID LAYER AND MEANS FOR PROJECTING INTO A SECOND IMAGE PLANE SAID PARTIAL IMAGES OF THE INDIVIDUAL COLOR SEPARATIONS SUPERIMPOSED ON ONE ANOTHER AND IN CORRECT REGISTER. 